1. Field of the Invention
This invention relates to polyisocyanurate foams. More specifically, it relates to rigid polyisocyanurate foams prepared from relatively storage-stable polyol compositions.
2. Background of the Art
Polyisocyanurate foams are widely used as insulating materials in the construction industry. Typically these foams are closed-cell, rigid foams containing a low-conductivity gas, such as a hydrochlorofluorocarbon (HCFC), in the cells. The foaming compositions, being liquids, may be used in pour-in-place applications; sprayed applications; and to form rigid foam boards. The boards, in particular, often include a facing, such as a metal foil, to which the foam adheres.
Unfortunately, polyisocyanurate foams, typically formed from polyester polyols and methylene diphenyl diisocyanate (MDI) at an MDI/polyol ratio over 250, may suffer from drawbacks. One frequent problem is that the foams may exhibit poor cohesion and/or adhesion (tensile bond strength) when faced with metals such as foils and steel. They may also suffer from formation of undesirable voids at the foam/metal interface.
For many polyisocyanurates, the polyester polyol employed is an aromatic-based structure. While such polyisocyanurate foams have many uses, it has been found that inclusion of at least some aliphatic polyester polyol may offer benefits. For example, US 2006/0047011 A1 discloses that polyisocyanurate foams prepared from aliphatic polyester polyols may exhibit improved flame resistance, lower thermal conductivity, reduced brittleness and improved surface adhesion. Low viscosity enables potential use in spray foams. The aliphatic polyols used therein are based on a combination of adipic, glutaric, succinic and nitric acids with water, esterified with ethylene glycol. These polyols have hydroxyl (OH) numbers of greater than 200 and viscosities in the range of 2,000 mPa*s.
In another example, a combination of aliphatic and aromatic polyester polyols is described in US 2001/0003758 as useful for preparing rigid isocyanurate-modified polyurethane foams. The foams have an isocyanate index ranging from 80 to 380.
Notwithstanding the above-described art, there is still a need for polyiso-cyanurate foams exhibiting improvements in foam cohesion and in adhesion to steel facings. These and other features may be found in the present invention.